Microcrystalline thin-film solar cells comprise various layers of silicon that differ in their doping and degree of crystallinity. The layers are deposited on substrates such as glass. Various methods are known for this purpose. In the PECVD method a gas mixture is decomposed into fragments in a plasma, and in the HWCVD method the gas or gas mixture is decomposed by heating a wire. In both methods decomposition products deposit on the substrate, resulting in a coating. The photo-CVD method and sputtering, known from the prior art, may also be used to deposit layers on surfaces.
A distinction is made between pin cells and nip cells, that differ in their layer sequence.
In a pin solar cell the layer adjoining the transparent substrate is a p-layer, i.e. a p-doped silicon layer; the i-layer is located on the p-layer and is not doped. The n-layer, that is n-doped, is located on the i-layer. In a nip solar cell the p-layer and the n-layer are exchanged with one another, and likewise include an i-layer.
The microcrystalline p-layer is p-doped silicon that is doped with boron, for example.
One skilled in the art is familiar with the manner in which these layers are deposited.
For a p-layer, gas mixtures containing silicon compounds, positively doped compounds such as boron compounds, as well as hydrogen gas are used that are decomposed and deposited on the surface. The resulting layer is microcrystalline. The known HWCVD, PECVD, and photo-CVD methods may be used for this purpose.
For the deposition of the microcrystalline i-layer, a mixture comprising a silicon compound and hydrogen is decomposed, whereupon a microcrystalline Si layer is deposited. The known HWCVD, PECVD, sputtering, and photo-CVD methods may likewise be used for this purpose.
For the deposition of the microcrystalline n-layer, a mixture comprising a silicon-containing compound, hydrogen, and negatively doped compounds such as phosphorus-containing compounds is decomposed. The HWCVD, PECVD, and photo-CVD methods may be used here as well.
According to one known method, thin-film solar cells are deposited onto transparent substrates by means of PECVD. The deposition sequence is pin or also nip, the individual layers generally being deposited in different cavities. For PECVD the microcrystalline layers are deposited by use of various possible excitation frequencies, such as radio frequencies, microwave, or very high frequency (VHF). High plasma outputs are necessary for achieving high deposition rates by means of PECVD. The HWCVD method requires high wire temperatures for achieving high deposition rates. Solar cells having a low fill factor and a low open circuit voltage have heretofore been produced at high deposition rates. The HWCVD method is also much more time-intensive than the PECVD method. Solar cells having very high Voc and FF may be obtained at low substrate temperatures.